Wandsworth council in south London failed to get an injunction against the National Association of Schoolmasters/Union of Women Teachers to prevent the boycott of all national curriculum tests, as well as forms of assessment the union believes 'unreasonable and unnecessary'.
Nigel de Gruchy, general secretary of the NASUWT, said it was 'a stunning victory', which would enable the union to continue its action to protect members against 'excessive workload'. Edward Lister, Wandsworth council's leader, said it would be seeking urgent legal advice on whether to appeal.
Tests next term for 14-year-olds will be worst affected by the boycott, since most of the union's members teach in secondary schools. But the High Court decision makes it highly likely that the other main unions - the National Union of Teachers and the Association of Teachers and Lecturers - could join a boycott, spreading the disruption to primary classrooms.
The NUT will vote next month on a boycott of this year's English tests for 14-year-olds, but it is already under pressure from its members to widen the action.
The ATL executive has up to now advised against a boycott, despite growing support from members for such a move against English tests for 14-year-olds. Both unions can expect stormy debates when they meet for Easter conferences.
Mr Justice Mansell ruled that the industrial action was a legal trade dispute under the terms of the Trade Union and Labour Relations (Consolidation) Act 1992, because the union's protest was 'wholly or mainly about workload or working hours'.
Wandsworth council had argued that the dispute was not about working hours but the national curriculum itself and was, therefore, illegal.
The judge said union members could not be in breach of their statutory duty by boycotting tests, as Wandsworth council had claimed. He said that under the 1988 Education Act, only headteachers, local authorities and governing bodies had statutory responsibilities for implementing the national curriculum, while teachers had merely contractual responsibilities to their employers.
The Department for Education responded to the ruling yesterday by reaffirming the importance of tests as 'the key to raising standards', but teachers will be hoping for a more conciliatory approach from John Patten, the Secretary of State for Education, when he addresses the ATL conference next week - the first time he has agreed to speak to a teaching union assembly.
Peter Smith, ATL general secretary, said: 'Wandsworth's failure to achieve an injunction cannot really be considered as a victory for anyone . . . . If the pressure for a boycott is to be avoided, Mr Patten must announce measures to ease teacher concern about the flawed nature of the tests and the heavy workloads.'
Industrial action could not provide a solution to the problems posed by the curriculum, which was 'unmanageable' in size and needed to be reduced to about 70 per cent of school time, Mr Smith said.
Doug McAvoy, NUT general secretary, said the High Court decision reinforced his union's intention to ballot on a boycott of English tests, but he remained cautious about extending the boycott to other subjects. 'We have always argued that our tactics must be to keep the issues focused. The conference will undoubtedly be looking at a wider boycott, but there is a danger that if you widen the focus, you blunt the instrument.'
David Hart, general secretary of the National Association of Head Teachers, said: 'Irrespective of one's views about whether action should be taken, I am glad that the judge has ruled this way because it does enable teachers to enter into a dispute if they are supported by an appropriate ballot.'
He predicted: 'The whole testing operation in primary schools this summer will collapse if the NUT widens the boycott, and if the ATL joins in, you can say goodbye to testing in secondary schools too.'
Mr Hart recognised that his own members had a statutory and a contractual duty to deliver the national curriculum in their schools, but suggested that heads who are sympathetic to the boycott might start pressing for action too.